1. Field of the Invention
This invention relates to a digital video signal recording apparatus, and more particularly to an apparatus for recording a variable-length coded digital video signal on a tape-shaped recording medium.
2. Description of the Related Art
In transmitting and recording a large amount of digital video data, it has been practiced to transmit and record the digital video data by compressing it through a high-efficiency coding process for efficient transmission and for reducing the amount of data to be recorded on a recording medium. For the high-efficiency coding process, it is a general tendency to adopt a variable-length coding method which excels in compressing efficiency.
The amount of information carried by digital video data handled, for example, by a high-picture-quality TV system which is becoming popular these days is five times as much as that of the data of the NTSC system. It is, therefore, being attempted to efficiently compress such a large amount of data by variable-length coding. Meanwhile, recording/reproducing apparatuses such as a digital VTR, a disc recording/reproducing apparatus and the like are now being arranged to be capable of recording data obtained by compressing such digital video data on a magnetic recording medium such as a magnetic tape.
However, in recording the digital video data in recording tracks on the recording medium after compressing the data by variable-length coding with the recording and reproducing apparatuses of the above-stated kind, the compressed amount of data obtained after compression by the variable-length coding is not uniform. It is, therefore, difficult to completely record the whole compressed data in each recording track which has a fixed recording capacity. The inconsistency of the compressed amount of data thus makes editing work of varied kinds such as tag recording and other special reproducing operations difficult.
To solve this problem, the code length of data can be adjusted by adjusting a quantizing parameter for the data to be variable-length coded, in carrying out the variable-length coding. For example, in a case where the variable-length coding is to be carried out by utilizing the known DCT (discrete cosine transform) process DCT transformed data is subject to the variable-length coding. It is known that, in this instance, the amount of data to be obtained after the variable-length coding is adjustable by adjusting a parameter to be used for quantization of the DCT transformed data.
The adjustment of the amount of data after the variable-length coding process can be made by using the DCT blocks as the units of adjustment. However, a change in the quantizing parameter brings about a change in the resolution of the applicable part. For example, an attempt to suppress the amount of the variable-length coded data to an amount below a given amount for every DCT block or for every picture, results in it being hardly possible to obtain a stably reproduced images, because of excessive fluctuations of picture quality among blocks or pictures.